Attribution of strong winds to a cold conveyor belt and sting jet

Abstract

A high‐resolution version of the WRF (Weather Research and Forecasting) model has been used to study the fine structure of a cloud head and its associated cold conveyor belt jet (CJ) and sting jet (SJ) in an intense extratropical cyclone that produced damaging surface winds in northern Ireland and central Scotland on 3 January 2012. The model was run with many different initialisation times and physical parametrisations, and a run was selected that verified well against a variety of observations. New methods have been devised to visualise the 3D structure of the CJ and SJ and to attribute strong surface winds to one or other of them, and the validity of regarding the SJ as a semi‐Lagrangian feature has been assessed. The model suggests that, whereas the CJ remained mainly below the 850 hPa level as it circulated around the bent‐back front, the SJ consisted of a stream or streams of air within the bent‐back frontal zone that first ascended from close to the surface into the middle and upper‐level parts of the cloud head before descending from evaporating cloud filaments at the tip of the cloud head and reaching the top of the boundary layer slightly ahead of the CJ. The simulations did not support the idea that either the evaporation or conditional symmetric instability (CSI) played a major role in the development of these jets. The strong gusts (up to 47 m s−1) which were recorded on the north coast of Ireland appear to have been due mainly to the CJ, which by then was undercutting the SJ. The SJ was responsible for stronger surface winds than the CJ several hours earlier during the initial stage of frontal fracture, but only for a limited period.

Cyclones, windstorms and the IMILAST project

“…Moreover, profiles even further west (not shown) indicated even colder low-level air (3.58C colder at 950 hPa 80 km from the 'west' sounding). Wind directions and temperatures at the surface also match the criteria put forward by Smart and Browning (2014) for distinguishing a SJ from a CJ. And those criteria were for the same case.…”

“…Model evidence across studies remains somewhat inconsistent however. For example, MartinezAlvarado et al (2010) found different vertical structures of jets in simulations of one case by two different models, whilst Smart and Browning (2014) found little evidence of an evaporative contribution in their simulations of Ulli. Indeed all previously published simulations, to the authors' knowledge, failed to resolve/show the ubiquitous SJ hallmarks one sees on imagery.…”

“…Recognising also the difficulties that models have in representing SJs and gusts in general, partly because of resolution (see Martinez-Alvarado et al, 2012, and Section 4 below) and partly because of boundary layer parameterisation issues (e.g. see Parton et al, 2009;Smart and Browning, 2014) we again bring observational evidence to the fore, specifically surface station data and satellite imagery. So for our purposes a SJ zone will be broadly defined to be: a small region affected by exceptionally high surface wind gusts, that occurred close to but downstream of the tip region of a cloud head of a cyclone that was undergoing, or had very recently undergone, rapid intensification.…”

“…Similarly, there is ordinarily a first clockwise wind shift on the main cold front, just before SJ arrival and a second clockwise wind shift, on the bent back cold front, after the SJ has passed. Smart and Browning (2014) discuss using these aspects to isolate the SJ for Ulli.…”

“…The tropical phases for the two ET cases were discounted. Smart and Browning (2014) performed high-resolution simulations for the Ulli case with the WRF (Weather Research and Forecasting) model. They found some evidence of a SJ in this simulation but indicated that the CJ was stronger, and that the maximum gusts at Malin Head, a very exposed site on the north coast of Ireland, were probably attributable to the CJ.…”